Sound recording



Jan. 1, 1935. J. G. FRAYNE 1,986,569

SOUND RECORDING Filed Oct. 8, 1930 INVENTOR J. GIRAVNE SOUND RECORDING .iohn GQ iirayne, fasade na, Calif" assignor to I lectrical Researchlroducts Inc., New York, N.}Y. a corporation of Delaware r I L l Application October 8, 1930, Serial No. 487,191

In'Great Britain September 4, 1930 r This mvenncnrelatesw sound recording and particularly to'a method of recording by photomeans a widerrange of sound volumes than can be recordedby known methods. In any method of recording sound the range of e v'oiiimesbf sound which can, be satisfactorily recorded isiimited by the properties of therecording medium used and the handling which the completed record receives. I The least fvolume which may berecordedis the Yvoluine which in reproduetion can be satisfactorily distinguished above the noise produced by irregularities in the structure'of the mediumsu'se'd in recording and reproducing and bythe dirt, scratches, etc. which collect on theirecord during-its useful life.

The object of this invention a system of sound recording willproduce a reduction of the volume of noise produced from a photographic sound record during the reproduction of the umes of sound to be satisfactorily recorded and thus 'incifeases thel range of volumes of sound which may berecorded. Or this invention will provide a larger ratiobetween the present minimumvolumeof sound. and the volume of noise producedduring reproduction. p I

Experiments have shownth-atrthe volume of noiseproduced during the reproduction or a photo'graphic sound record is approximately proporclonal-ta the average transmission of the positive sound-record. The average transmission of; a photographic sound'record is consideredfto be portions of thesound-record. The transmission of a photographic medium is defined as the ratio of' 'the' lighttransmitted through the medium dii'rided by theglight incident on the medium and for conve nience is commonly expressed as a per- "centageb 4 A 'featureofthis invention isa method of producing a positive photographic sound record in vvhich the average transmission oi the sound record varies with the volume of the sound recorded. "for small volumes ofv sound a dense-record is produced and the noise produced during reproduction'is aminimum. t

-In the reproduction of sound from a photo graphic sound record it has been shown thatthe volume of the reproduced sound is proportional to theproductof the average transmission of the so und'record multiplied by the percentage variation in this transmission due to the modulation of the transmission accordance with the auna; In order to prevent distortion of the volume-of sound from the "record. This reduction in the:

equal to the 'transmissionbf theunmodulated A photographic positive is an inverse {copy of the photographic negative from; which the positive was printed thus if "the positive sound record has a small average transmission the negative sound re'co'rdmust have a correspondingly large transmission. The exposure to which'a photographic material is exposed is defined as proportional to the product of the intensity of the light incident ion the material multiplied by the time during .which the material is subjected to this light; if the completephotograph-ic process is assumed to be controlled so; that no distortion is produced, then the transmission of the positive sound record will be proportional to the eirposure of the negative. Accordingly, toprodu'ce a positive "sound record having 'asniall average transmission, the exposure of the negative must be 001'- responding-1y small. In accordance with this inventi'onthe time of exposure is reduced toproduce a negative sound record having alarge average transm ssion.

A further feature of thisinvention is-a method of recording sound by photographic means in which thetime of exposure of the negative sound record varies directly with volume of the sound recorded and'the intensity of the recording light is virtually completely modulated forall volumes of sound.",

In a preferred embodimentof this invention, a lightvalve of-the type disclosed in U S; Patent 1,638,555 granted August 9, 1927 toE. C. Wente. is employed. In using a light valve of thistype for-recording sound by the method disclosed in the above patent the intensity of "the recording light does not change but thetime of exposure is varied as the height of the slot defined by the ribbons is varied by the movement of the ribbons. The average time of exposure will be controlled by the average spacing of the ribbons or, in other words, by the height of the slot formed when the ribbons are not displaced by a current flowing in them. In accordance with the present invention a constant unidirectional current is applied tot hie ribbons in such direction andofsuch magnitudethat the ribbons areiifearly touching and the height of the slot "defined by the ribbons is varies in accordance with the volume of the sound.

The output of the rectifier is supplied to the ribbons of the light valve in a direction to oppose the steady current, and cause the ribbons to move apart. The height of the slot defined bythe ribbons is thus enlarged in direct. proportion to the volume of the sound being recorded. At the same time the ribbons are vibrated by the amplified currents from the sound detectorand vary the time of exposure of'the film in accordance with the modulation produced by the sound. As the amplitude of the vibrations is controlled by the volume of the sound, and as theheight of the slotis simultaneously controlled by rectified currents varying with the volume of the sound it will be apparent that the slot is-always just large enoughzto allowthe vibration. In other-words, the light passing through the valve is virtually I completely modulated for-all volumes of sound. Also as the slot is always opened just enough toallow for the vibrations, aloud sound cannot overload the valve-and cause the ribbons to clash together.- In the case of a very large volume of sound, the rectified current may be larger than .the steady current and cause the ribbons to be forced apart so that the height of the slot is greater than the normal spacing of the ribbons with no current applied. A larger volume of soundmay thusbe recorded by the method of this ;invention than can be recorded by known methods.

-A furtherfeature of this invention is the provision of means whereby the ribbons of the light valve areprevented from clashing together when the valve is'overloadedby a large volume of sound.

It is welllknown that the definition of a photo- "graphic sound record is improved when the recording slot is made very small, and due to the finite height of the recording and reproducing slots, there isa loss of definition and power in the reproduction of the higher frequencies of sound. Measurements have shown that for equal efiects on the ear much more power is required in a low frequency sound than in a high frequency sound.

. Thus in the present systemif we consider two sounds having equal effect on the ear,one sound of-lowfrequency and the other sound of high frequency, the'low frequency sound will be of average power or volume and the ribbons of the light valve will be opened to normal spacing producing a record having as good definition as the records produced by previously known methods. The high frequency sound will be of low volume and as the ribbons of the light valve will be nearly closed, the definition of the record of the high frequency sound will be better than the records produced heretofore and will partially offset the loss due to the finite height of therecording slot.

In the drawing f Fig. 1 represents in schematic form a preferred embodiment of the invention; and

f Fig.2 represents diagrammatically a film sound recording system.

InFig. 1 sound waves are detected bya known type of microphone 10 and cause the current from thelbatter'y 11 to. bemodulated in accordance withsouiid. The sound modulated currents in the primary winding of transformer 12 induce similar currents in the secondary winding of the transformer. The output of transformer 12 is amplified in a known type of amplifier 13. Part of the output of the amplifier 13 is applied through a transformer 14 to the movable elements 15 of a light valve which for convenience is shown as a modification of the light valve disclosed in U. S. Patent 1,638,555 August 9, 1927to E40. Wente. The modulated speech currents cause the movable elements of the light valve to vibrate in accordance; with the current.

In Fig. 2 the image of the filament of a suitable lamp 16 is focussed by the lens 17 on the small slot formed by the movable elements 15 in the orifice 18 pierced in the pole pieces of a permanent magnet 19. Suitable pole faces concentrate the magneticfield'o'f the magnet 19 to form a uniform magnetic field in which the movable elements 15 are immersed. An electric current flowing in the-movable elements 15 will cause the elements to be displaced by the reaction of the magnetic fields due to the current and the magnet. The image of the slot is focussed by the lenses 20 and 21 on a photographic film 22 moving at uniform'speed past the point of exposure. An aperture plate 23 holds the film smooth at the point of exposure and prevents diffused light from fogging the record.

. In Fig. 1 a portion of the sound modulated currents from the amplifier 13 is applied through a tapped transformer 24 to the grid of a vacuum tube 25. The bias applied to the grid of the vacuum tube 25 may be varied by means of the potentiometer 26. The'sound modulated input to the grid of-the vacuum tube 25 may be varied by means of the taps brought out from the secondary of the transformer 24. The plate current of the vacuum tube may thus be adjusted to any convenient value and the sound modulated inputadjusted to obtain the highest coefiicient of detection. Plate current is supplied to the vacuum tube 25 from a battery 27 through the coupling resistances 28 and 29 which may conveniently be in the form of a tapped resistance. The combined resistances 28 and 29 are of suitable value to assist in producing complete detec-. tor action in the vacuum tube 25.

. Thedetected currents in the output of the vacuum tube 25 produce a pulsating voltage across the resistance 29 which is applied to the grid-filament circuit of a vacuum tube 42. A steady bias is applied tothe'grid of the vacuumtube 42 by a battery 32 and the valueof this bias may be adjusted bythe potentiometer 31 shunted across the battery 32. I I

' The grid of vacuumtube 42 is connected to the junction of the resistances 28 and 29 and the wire 43 from the biasing potentiometer 31 is connected to the plate of the vacuum tube 25. The steadycomponent of the plate current of the vacuum tube 25 flowing in. the resistance 29 will produce a voltage drop across the resistance 29 which reduces the bias on the grid ,of'the vacuum tube 42. Thus when sound modulated currents are detectedby the vacuum tube 25, the bias on the grid of vacuum tube 42 will be reduced and the plate current of vacuum tube 42 will increase. A choke coil in the output circuit of the 'vacuum tube 25 cooperates with a condenser 51 connected across the output circuit to form a smoothing Y circuit. The smoothing circuit causes the voltage variations developed across the resistance 29 to follow-the envelope of the amplitude variations of the sound modulated currents;

and not the instantaneous variations in amplitude.

Current froma battery 44 is applied through a l variable resistance 45 to the movable elements 15 of the light valve and cause the movable elements to reduce the slot to a small value which may be adjusted by means of the resistance 45. The plate current of the vacuum tube 42 flows from the battery 46 through the vacuum tube 42, wires 4'7 and 48, and the movable elements 15 to the battery 46. This current will flow in a direction to oppose the current from the battery 44 flowing in the movable elements 15 l and to tend to cause the movable elements 15 to open the slit. A high resistance ,49 permits the 1 plate current flowing through the movable elements to be adjusted.

The operation of the circuit shownin Fig. 1 is as follows: When no sound modulated current is applied to the circuit, the bias applied to the i grid of the vacuum tube.42 is adjusted so that i the plate current flowing in the movable elements 15 is very small and the current from the bat--,.;; tery 44 flowing in the movable elements 15 is adjusted so that the slot defined by the movable elements 15 is small. When sound modulated of the current from the battery 44 and permit the movable elements 15 to move apart and open the slot. Simultaneously the sound modulated currents from the transformer 14 cause the movable elements 15 to vibrate. By suitably adjusting the inputto the vacuum tube 25 the slot may be opened just enough to allow the full amplitude of the vibration of the movableelements 15 and substantially complete modulation of the light reaching the film will be attained for all 1 volumes of sound. A condenser 52 connected across the output of the vacuum tube 42 causes the vacuum tube 42 to respond more rapidly to changes in amplitude of the sound modulated currents so that the slot in the light valve is quickly increased in height when a sudden loud sound is detected by the microphone 10. When the volume of sound issufiiciently large the plate current from vacuum tube 42 flowing in the movable elements 15 will equal the current from battery 44 flowing in the movable elements 15 and the height of the slot will be determined by the mechanical adjustmentgiven to the movable elements 15. This is the maximum volume which i may be recordedby known systems of light valve 44 flowing in the movable elements 15 and to force the movable elements 15 apart so that the mean spacing of the elements is greater than the spacing dueto the mechanical adjustment.

The light valve can thus record a volume of sound which in known types of light valvewould ordinarilyoverload the valve and cause the movable elements to clash together.

While the inve tion has been applied to a speto the specific type of light valve disclosed.

What is claimed is: 1. In a system for recording modulated currents on a moving film, in combination, a source of light, alight valve actuated by modulated currents to vary the light from said source incident on the film, a detector energized by modulated currents, a thermionic amplifier having its controlelectrode connected to the output of said detector and having the light valve connected in serial relation in its output circuit, a source of constant current in series with a variable resistance connected in shunt with the light valve to oppose the output current flowing in said light valve, and a second variable resistance in parallel relation with said source of current whereby the of light, a light valve actuated by modulated currents to vary the light from said source incident on the film, a detector energized by modulated currents, a. thermionic :amplifier comprising a cathode, an anode and a control electrode, means for applying a voltage varying withthe output of said detector between the cathode and the control electrode of said amplifier, a source of current in serial relation with said light valve connected between the anode andthe cathode of said amplifier, a second source of current in serial relation with a variable, resistance connected in shunt with said light valve to oppose the current from said first source fiowing'in the light valve,

and a,second variable resistance in shunt with said second source of current, said second readjustment of the current flowing in the light valve.

3. In a system for recordingv modulated currents on a light-sensitive medium, in combination, means for directing a beam of light on said medium, a light valve having movable elements mechanically adjusted to define an image of said beam of normal height in the direction of motion of said medium, a circuit for supplying modulated currents, to vibrate said movable elements, means for applying a constant current to said elements to decrease the height of said normal image, a detector energized by said modulated currents, a resistorin the output circuit of said detector, a thermionic amplifier having a cathode, an anode, and a control electrode, a source of current in serial relation with the movable elements of said light valve connected between said cathode and said anode so as to oppose the constant current in said elements, a source of biasing potential inserial relation with said resistor connected between said cathode and said control electrode and adjusted so that in the absence of modulated currents the anode current of said amplifier is substantially zero, said resistor being so connected that the potential difierence acrosssaid resistor tends to reduce said biasing potentialwhereby in the presence of modulated currents, theanode current of said amplifierreduces the effect of the constant current in said elements, and at times causes said elements to increase said image to a height greater than the height due to the mechanical adjustment.

JOHN G. FRAYNE.

sistance being operated simultaneously with said 

